The present invention relates to a process for producing a SiC whisker having a surface coated with BN (boron nitride) and a process for producing a fiber-reinforced ceramic (FRC) wherein use is made of the BN-coated SiC whisker as a reinforcement.
Ceramic materials have been used as various structural members to be used at a high temperature, including parts of engines, by virtue of their excellent heat resistance, high-temperature strength, chemical stability, etc. The ceramic materials, however, are generally poor in the fracture toughness and have an inherent drawback that the occurrence of even small flaw and internal defect causes stress to be concentrated, so that they are apt to fracture.
For this reason, in recent years, studies have widely been made on a method of improving the fracture toughness of ceramic materials, and many proposals have been made on an attempt to improve the toughness through the use of a SiC whisker as a reinforcement of ceramic materials. The purpose of these proposals is to improve the fracture toughness through function exerted by dispersing a SiC whisker having a high elasticity in a ceramic to form a composite material, such as the termination or inhibition of the growth of cracks in the ceramic, the relaxation of the stress concentration by virtue of the deflection of the cracks, and the energy absorption at the tip of the cracks by virtue of a pull-out effect of the whisker.
The composite effect attained by the use of the SiC whisker as a reinforcement is greatly influenced by an interfacial bonding between the SiC whisker and the ceramic matrix. Specifically, when the interfacial bond is strong, the above-described function of improving the toughness cannot be sufficiently exhibited, while when the interfacial bond is weak, this portion unfavorably becomes starting point of the cracks.
An excessive interfacial bond between the SiC whisker and the ceramic material as the matrix is mainly due to the occurrence of a chemical reaction at the interface of the both during sintering. Therefore, if the surface of the SiC whisker is previously modified so that the chemical reaction hardly occurs, the interfacial bonding is relaxed and the above-described function of improving the toughness can be effectively attained.
This has led to the development of processes for producing a ceramic composite material wherein SiC whiskers having a surface coated with various unreactive substances are used as a reinforcement (see, for example, Japanese patent application Kokai publication Nos. 62-12671, 63-182258, 63-277566, 64-3081 and 1-115877.
In order to effectively improve the fracture toughness through the use of a surface-modified SiC whisker, besides the chemical unreactiveness of the coating substance with the matrix, the surface of the whisker should have a lubricity or cushioning property for promoting the pull-out effect. A suitable substance capable of meeting both the above-described requirements is BN.
Meanwhile the following methods have been proposed for forming a BN coating on the surface of the SiC whisker:
(A) a method described in Japanese patent application Kokai publication No. 62-12671,
(B) a method described in Japanese patent application Kokai publication No. 63-182258, and
(C) a method described in Japanese patent application Kokai publication No. 63-277566.
The method (A) comprises chemically depositing BN on the surface of a SiC whisker through the use of a mixed gas comprising B.sub.2 H.sub.6 (diborane) and NH.sub.3. Since, however, B.sub.2 H.sub.6 is a substance which readily ignites in the air, this method is problematic in that the operation of BN deposition is accompanied with danger.
The method (B) comprises forming a BN coating through the thermal decomposition of B.sub.3 N.sub.3 H.sub.6 (borazole). Since, however, borazole is used in a high concentration (10%), this method is problematic in that the thickness of the BN coating remarkably varies in a range of 0.02 to 0.3 .mu.m and the thickness is too large to improve the fracture toughness of the ceramic.
The method (C) comprises chemically depositing BN through the use of a mixed gas comprising boron chloride, NH.sub.3 and H.sub.2. This method is disadvantageous in that the procedure is complicated and it is difficult to control the thickness of the BN layer.